We usually use 1 2 3 4 5 6 7, the seven sounds in the aria (singing) are called roll call, and the seven letters of C D E F G A B correspond to seven sounds, which are called sound names, so the sound name of roll call 6 (la) is A, and the frequency is 440HZ, and vice versa. Usually both names staff acceptable. For a violin, the pitch of the empty string of the second string is 6(la), which is a sound. In addition, in a band, A sound is used as a standard sound to adjust the pitch of various instruments. For reference.